Multiple stressors affect function rather than taxonomic structure of freshwater microbial communities

Microbial community responses to environmental stressors are often characterised by assessing changes in taxonomic structure, but such changes, or lack thereof, may not reflect functional changes that are critical to ecosystem processes. We investigated the individual and combined effects of nutrient enrichment ( + 10 mg/L N, + 1 mg/L P) and salinisation ( + 15 g/L NaCl)-key stressors in freshwater systems-on the taxonomic structure and metabolic function of benthic microbial communities using 1000 L open freshwater ponds established >10 years ago in the field. Combined stressors drove strong decreases in maximum and mean total carbon metabolic rates and shifted carbon metabolic profiles compared to either stressor individually and compared to ambient conditions. These metabolic functional changes did not recover through time and occurred without significant alterations in bacterial community taxonomic structure. These results imply that critical functions, including organic carbon release, are likely to be impaired under multiple stressors, even when taxonomic structure remains stable.